1. Field of the Invention
The present invention relates to a sheet-like switch provided with a plurality of switch devices.
2. Description of the Related Art
A typical sheet-like switch of a known type is constructed as shown in FIG. 5. Namely, an upper electrode sheet 1 is formed by printing predetermined upper contacts 2 and upper conductive patterns 3, both of which are made from a silver paste, on an insulating sheet, such as polyester film or the like. An overcoat 4 formed from resist ink, carbon ink or the like, is further disposed on the sheet 1 to cover the conductive patterns 3 except for the upper contacts 2 positioned at the ends of the respective patterns 3. A spacer film 5, formed of polyester film or the like, is provided with holes 5a in the predetermined positions. A lower electrode sheet 6, as well as the upper electrode sheet 1, is formed by printing lower contacts 7 and lower conductive patterns 8, which are made from a silver paste, on polyester film or the like. An overcoat 9 similar to the overcoat 4 is disposed on the sheet 6 to cover the lower conductive patterns 8 except for the lower contacts 7 located at the ends of the respective patterns 8.
With this construction, the upper and lower electrode sheets 1 and 6 are, as illustrated in FIG. 6, stacked on each other across the spacer film 5, and the upper and lower contacts 2 and 7 of the upper and lower conductive patterns 3 and 8, respectively, are located to opposedly face the holes 5a formed in the spacer film 5. In this manner, the sheet-like switch provided with a plurality of switch devices is constructed.
In the sheet-like switch of the above known type, as shown in FIG. 7, the upper and lower conductive patterns 3 and 8 located within the hole 5a of the spacer film 5 are extended to the exterior in the same direction and overlaid on each other across the spacer film in the upward and downward direction. Further, the thickness of the spacer film 5 is as thin as approximately 75 .mu.m, so that the upper and lower conductive patterns 3 and 8 are positioned in very close proximity to each other.
Hence, as illustrated in FIG. 8, the absence of the overcoats 4 and 9 causes silver to precipitate from the upper and lower conductive patterns 3 and 8 placed in proximity to each other in the hole 5a of the spacer film 5. This further encourages the generation of a silver-migrating portion 10 using the inner wall of the peripheral portion 5b of the hole 5a as a medium, which may establish an electrical connection between the conductive patterns 3 and 8. The provision of the overcoats 4 and 9 thus serves to prevent this electrical connection.
The switch constructed as described above is operable as follows. The upper contact 2 on the upper electrode sheet 1 is pressed by a finger or the like. This deflects the sheet 1 to connect the upper and lower contacts 2 and 7 overlaid in the holes 5a across the spacer film 5, thereby turning on the switch. On the other hand, the upper contact 2 is released to separate from the lower contact 7, thereby turning off the switch.
However, as described above, the sheet-like switch of the above conventional type is constructed in such a manner that the upper and lower conductive patterns 3 and 8 are extended to the exterior in the same direction and overlaid on each other across the spacer film in the upward and downward direction. As the switch is becoming thinner, the upper and lower conductive patterns 3 and 8 are located in closer proximity to each other, which encourages the generation of migration, which may establish an electrical connection between the conductive patterns 3 and 8.
In order to take measures against this migration, the provision of the overcoats 4 and 9 on the upper and lower conductive patterns 3 and 8, respectively, is conventionally required. This disadvantageously increases the manufacturing processes and cost.